1. Field of the Invention
The present invention relates to a power transmission mechanism to be used as a torque limiter for use in a compressor, general industrial equipment and so forth.
2. Description of the Related Art
First, a power transmission mechanism (hereunder sometimes referred to as a first conventional mechanism) for use in a compressor, which is described in Japanese Examined Utility Model Publication (JP-Y) No. 39105 of 1994, will be described hereinbelow by referring to FIGS. 1A to 1C.
A pulley 25 is rotatably mounted on a front nose 23 of a housing 22 of a compressor 21 through a bearing 24. Further, a rotation transmission plate 27 is fixed to a shaft 26 of the compressor 21. Moreover, synthetic resin destructible elements 28, which may be destructed by overload, are fixed to the rotation transmission plate 27 at four places, respectively. Incidentally, an end portion of each of the destructible elements 28 is inserted into a corresponding one of holes 25A respectively bored in the pulley 25 at the four places.
As a result of configuring the mechanism in this manner, when an abnormality occurs in the compressor 21 and a torque, whose magnitude exceeds a predetermined value, is applied to the rotation transmission plate 27, the destructible elements 28 are destructed. Thus, power is not transmitted from the pulley 25 to the rotation transmission plate 27. Consequently, the power transmission mechanism is protected.
Next, another power transmission mechanism (hereunder referred to as a second conventional mechanism) for use in a compressor, which is described in Japanese Unexamined Utility Model Publication (JP-U) No. 142460 of 1988, will be described hereunder by referring to FIGS. 2A and 2B.
A pulley 35 is rotatably mounted on a front nose 33 of a housing 32 of a compressor 31 through a bearing 34. Further, a hub 37 is fixed to a shaft 36 of the compressor 31. Moreover, each of drive levers 39 is attached to the hub 37 at a corresponding one of four places in such a way as to be able to rotate around a corresponding one of rivets 38. Furthermore, four engaging recess parts 37A are provided in the outer peripheral portion of the hub 37. A ring-like metallic plate spring 40 is disposed on the outer periphery of the hub 37 in such a manner as to be fit into the engaging recess parts 37A, respectively. An inner-end round engaging portion 39A of each of the drive levers 39 engages a corresponding one of the engaging recess parts 37A of the hub 37 through the plate spring 40. Further, an outer-end round engaging portion 39B of each of the drive levers 39 engages a corresponding one of engaging recess parts 35A of the pulley 35.
As a consequence of such a configuration of this mechanism, when an abnormality occurs in the compressor 31 and a torque, the magnitude of which exceeds a predetermined value, is applied to the hub 37, each of the drive levers 39 rotates around a corresponding one of the rivets 38. Thus, each of the inner-end round engaging portions 39A disengages from the corresponding one of the engaging recess parts 37A of the hub 37. Moreover, each of the outer-end round engaging portions 39B disengages from the corresponding one of the engaging recess parts 35A of the pulley 35. Therefore, power is not transmitted from the pulley 35 to the rotation transmission plate 37. Consequently, the power transmission mechanism is protected.
Furthermore, still another power transmission mechanism (hereinafter sometimes referred to as a third conventional mechanism) for use in a compressor, which is described in Japanese Unexamined Patent Publication (JP-A) No. 135752 of 1996, will be described hereunder by referring to FIGS. 3A and 3B.
An inner ring of a ball bearing 53 is fixed on a cylindrical projection portion 52A of a front housing 52 of a compressor 51. Further, a rotor 54 is fixed to the outer ring of the ball bearing 53. Moreover, a pulley 55 is fixed to the rotor 54. Furthermore, pins 56 are press-fitted into and fixed to holes 54A bored in the rotor 54 at three places located on the circumference x thereof. An elastic ring element 57 made of various kinds of rubber is fitted into and fixed to each of the pins 56. A second hub 61 is secured by three rivets 62 to the flange 59A of a first hub 59 which is fixed to a shaft 58 of the compressor 51 with a nut 60. Holding members 63 are secured onto the second hub 61 at three places, respectively. Further, each of the holding members 63 consists of a pair of holding pieces 63A and 63B. Further, the interval A between the exit-opening-side portions, which are placed forward in the direction R of rotation of the pulley 55, of the holding pieces 63A and 63B of each pair is set in such a manner as to be narrower than the interval B between the entrance-opening-side portions, which are placed rearward in the direction R, of the holding pieces 63A and 63B of the same pair. Moreover, each of the elastic ring elements 57 is sandwiched by a corresponding pair of the holding pieces 63A and 63B.
When the compressor 51 is normally operated, power is transmitted from an engine through the pulley 55, the rotor 54, the pins 56, the elastic ring elements 57, the second hub 61 and the first hub 59 to the shaft 58. Thus, the compressor 51 operates. At that time, the elastic ring elements 57 also perform the action of absorbing a variation in torque.
In contrast, when an abnormality occurs, for example, when the compressor 51 is locked, each of the elastic ring elements 57 is elastically deformed and passes through an exit-side opening part formed between the exit-opening-side portions of the pair of the holding pieces 63A and 63B. Thus, the transmission of power is interrupted.
The aforementioned first conventional mechanism has drawbacks in that repeated stress is produced in the destructible elements, which are destructed by overload, by driving the compressor, that it is, thus, difficult to maintain a destruction torque at a constant value and that the destruction torque decreases with time.
The aforesaid second conventional mechanism has drawbacks in that the number of components thereof is large, that the configuration thereof is complex, that because the drive levers are long, it is difficult to reduce a pulley diameter, and that because bending stress is applied to the drive levers by driving the compressor, it is difficult to suitably design the drive levers.
In the case of the aforementioned third conventional mechanism, when the torque excesses the predetermined value, each of the elastic ring elements is deformed and passes through the exit-side opening part formed in the holding member. Thus, the power transmission is interrupted. However, the loci of the elastic ring elements are on the same circumference. Thus, even when an elastic ring element once passes through the exit-side opening part of the holding member, another elastic ring element may enter the holding member from an entrance-side opening thereof. As a result, some power is transmitted to the shaft. Therefore, the reliability in interrupting the transmission of power is low. Further, each time when each of the elastic ring elements is brought into contact with and is detached from the holding member, noises and vibrations are generated. Moreover, because it is difficult for each of the elastic ring elements to enter the holding member from the exit-side opening thereof, power can be transmitted only in an orientation.